The present invention relates generally to processing of spent fuel rods and, more particularly, to accelerator-driven transmutation of spent fuel elements such that actinides and long-lived fission products are transmuted, and electrical power and new fuel are generated.
Present nuclear waste strategies, centered about geologic repository storage, require geologic stability and separation of wastes from human contact for tens of thousands of years. Transmutation offers the potential for transforming the time scales associated with such storage to hundreds of years or less.
Transmutation of long-lived nuclear wastes to short-lived or stable isotopes has been studied for many years. A sampling of illustrative techniques is presented in xe2x80x9cA Conceptual Study of Actinide Transmutation System With Proton Accelerator: (1) Target Neutronics Calculation,xe2x80x9d by H. Takada et al., Proceedings Of The 2nd International Symposium On Advanced Nuclear Energy Research-Evolution By Accelerators, Jan. 24-26, 1990 Mito, Ibaraki, Japan. Therein, the authors describe a transmutation apparatus using KeV neutrons which requires large material inventories to achieve significant transmutation rates since cross sections for neutron capture are small at these neutron energies. Moreover, the proton beam is admitted to the subcritical reactor target using a window, which limits the neutron flux available for the process. The direct interaction between the proton beam and the sodium coolant will produce substantial quantities of oxygen, carbon, nitrogen, and hydrogen spallation product which may combine to generate tar. Finally, degradation of the cladding material for the nuclear waste as a result of proton bombardment may present a lifetime problem. In xe2x80x9cAccelerator Molten-Salt Breeding And Thorium Fuel Cycle,xe2x80x9d by Kazuo Furukawa et al., Proceedings Of The 2nd International Symposium On Advanced Nuclear Energy Research-Evolution By Accelerators, Jan. 24-26, 1990, Mito, Ibaraki, Japan, the authors describe a windowless apparatus accepting high proton beam currents having GeV energies which are caused to impinge directly on the target materials as in Takada et al., supra, except cooled by molten salt. Transmutation is achieved using keV neutrons where the low cross sections of the neutrons require large inventories to achieve useful transmutation throughput. Additionally, since the thorium is mixed with lithium fluoride, proton spallation will again produce bothersome tars.
In xe2x80x9cStatus Report Of The SIN Neutron Source,xe2x80x9d by G. Atchison and W. E. Fisher, Proceedings Of International Collaboration On Advanced Neutron Sources (ICANS-VII), Sep. 13-16, 1983, Atomic Energy Of Canada, Limited, Report AECL-8488, the authors disclose a low-power target for low flux neutron production in Pbxe2x80x94Bi from neutron bombardment with subsequent neutron thermalization using heavy water. Heat is removed from the target by thermal convection, and the low power levels also permit the use of a window between the accelerator vacuum and the target. The proton beam strikes the target from below which has advantages for thermal convection cooling.
In xe2x80x9cApparatus For Nuclear Transmutation And Power Production Using An Intense Accelerator-Generated Thermal Neutron Flux,xe2x80x9d by Charles D. Bowman, U.S. Pat. No. 5,160,696, which issued on Nov. 3, 1992, the teachings of which are hereby incorporated by reference herein, high thermal neutron fluxes generated from the action of a high power proton accelerator on a spallation target allows the efficient burn-up of higher actinide nuclear waste by a two-step process. Additionally, rapid burn-up of fission product waste for nuclides having small thermal neutron cross sections, and the practicality of small material inventories while achieving significant throughput derive from employment of such high fluxes. The apparatus includes an accelerator, a target for neutron production surrounded by a blanket region for transmutation, a turbine for electric power production, and a chemical processing facility.
Accordingly, it is an object of the present invention to efficiently and subcritically transmute plutonium, higher actinides and long-lived fission products in nuclear waste to reduce the threat of proliferation and to minimize waste streams.
Another object of the present invention is to produce newly-enriched uranium for fuel reconstitution for use in base-load, conventional nuclear reactors.
Yet another object of the invention is to efficiently and subcritically transmute plutonium, higher actinides and long-lived fission products in nuclear waste while producing power to offset operational costs.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for processing spent fuel from nuclear reactors hereof includes the steps of: dissolving the spent fuel in a molten salt, forming thereby a solution; separating uranium and transition metals from the solution; exposing the solution to an intense flux of thermal neutrons; and separating the transmutation products for storage thereof; whereby plutonium and higher actinides, and fission products are transmuted.
In another aspect of the present invention, in accordance with its objects and purposes, the method for processing spent fuel from nuclear reactors hereof includes the steps of: converting the spent fuel to elemental metals; electrorefining uranium and thorium onto an electrode, thereby removing uranium and thorium from the elemental metals; electrorefining transuranic elements into a molten bismuth cathode, thereby removing transuranic elements from the elemental metals, blending the transuranic elements dissolved in molten bismuth with molten lead, or placing the transuranic elements dissolved in molten bismuth in canisters; exposing the blended solution or the canisters to an intense flux of fast neutrons; and separating the transmutation products for storage thereof, whereby plutonium and higher actinides, and fission products are transmuted.
Three ATW-NCC (Accelerator Transmutation of Wastes-Nuclear Cycle Closure) options are described, one of which will be pursued depending on whether nuclear power is to be eventually phased out (ATW-NCC1), continued at the present level or moderately increased levels (ATW-NCC2), or substantially expanded (ATW-NCC3). Functionally, these options are approximately equivalent to the options of once-through cycling, multiple reprocessing and full-fledged breeding, respectively, envisioned for nuclear power systems using current technology.
The three ATW-NCC options utilize spent fuel generated by any existing and conceivable future types of nuclear reactors. ATW-NCC""s front-end processes produce a feed of unseparated actinides suitable for nearly complete subcritical burning in the actinide burn unit which is driven by a large-current linear accelerator (LINAC). Power production to offset operational costs is optional.
In ATW-NCC1, excess neutrons generated by the accelerator and the fission of the higher actinides are used in a blanket/reflector containing long-lived fission products to be transmuted. The separated uranium is collected and sent to permanent storage. ATW-NCC1 requires the smallest LINAC driver. Plutonium and higher actinides are completely eliminated, as well as the most troublesome fission products. Uranium is not processed.
In ATW-NCC2 and 3, because of the deep subcriticality, a large number of neutrons is available to breed new 233U in a surrounding thorium blanket. This 233U is used to enrich uranium previously extracted from the spent fuel, and new fuel can be fabricated for use in base-load (power) reactors. ATW-NCC2 supported reactors will use uranium fuel. Reactors supported by ATW-NCC3 systems will use denatured thorium fuel, allowing a better fuel utilization factor at the expense of more complex process chemistry. Plutonium and higher actinides are removed from circulation after being generated in the base-load reactors: they are destroyed in their first (and only) pass through the ATW Actinide Burn apparatus and never recycled. About ⅓ of the uranium and thorium energy content is utilized in ATW-NCC2 (to use it completely would require a three-times larger accelerator or a smaller number of supported base-load reactors). The introduction of thorium in the base-load reactor fuel (ATW-NCC3) will eventually allow full utilization of the uranium and thorium energy resources with minimum accelerator size and a maximum number of supported base-load reactors. Both thermal (based on molten salt fuel) and fast-spectrum (based on liquid lead-bismuth fuel) are possible for the ATW Actinide Burn apparatus, with a significant neutronic advantage being gained for the ATW-NCC2 and -3 systems by the adoption of the very hard spectrum liquid metal ATW burner.
Advantages of the present invention include the elimination of plutonium, higher actinides and selected fission products from the nuclear waste stream. With the implementation of ATW-NCC2 and 3-type systems, efficient (and eventually full) utilization of the existing uranium and thorium energy resources will become possible. Additional advantages include the acceptance of nuclear spent fuel from any existing type of nuclear reactor, the prevention of plutonium accumulation, proliferation and diversion at all levels, full compatibility within the existing nuclear infrastructure of base-load reactors, and reduction of the volume of nuclear waste to be permanently stored (including uranium, and thorium in the ATW-NCC2 and 3 concepts) by a factor of greater than 100.